Summary Animals have an intrinsic ability to respond to threats in their environments, but the underlying mechanisms are poorly understood. A complete understanding of these complex stress-induced behaviors requires the characterization of all participating neurons, their connections, and their interactions with other tissues (including sympathetic connections in the gut, the circulation system, muscles, etc.). However, this level of analysis is difficult to achieve in complex vertebrate organisms. One rational approach is to analyze these processes in simpler invertebrate models. This proposal aims to understand the neural mechanisms that encode threat responses (both behavioral and physiological) in an invertebrate model system. The nematode, Caenorhabditis elegans, provides a unique opportunity to analyze the genes, cells, and circuits that regulate complex behaviors. The Chalasani lab has developed a novel model of threat behaviors that involves interactions between C. elegans and a second predatory nematode species, Pristionchus pacificus. A starving P. pacificus will attack and devour a C. elegans in 30 minutes. C. elegans in turn, seeks to avoid P. pacificus and its secretions. The Chalasani lab has characterized a novel, redundant neural circuit that detects the P. pacificus predator and drives rapid avoidance behavior, which entails a reversal in locomotion followed by a wide-angle turn. In addition to this rapid avoidance, the lab also discovered that C. elegans exposed to predator secretions for a long period of time (30 minutes) exhibit slowed locomotion (freezing), reduced egg- laying behavior, and the induction of mitochondrial stress in multiple tissues. These responses last up to one hour after the predator cue is removed, and are reminiscent of defensive behaviors observed in other predator- prey models. A pilot genetic screen identified seb-3 (the C. elegans homolog of corticotrophin releasing factor receptor 1 (crfr1)) as required for these long-term behavioral and physiological changes. This is the first evidence that CRF signaling affects behavior and physiology in response to an external threat in an invertebrate. Additionally, a cell culture assay system was used to identify a cognate ligand, NLP-49, that activates the SEB-3 receptor. Here, genetic methods will be used to characterize the role played by CRF signaling in coordinating behavioral and physiological changes in response to an external threat. Aim 1 will probe the role of CRF signaling components (the SEB-3 receptor, the NLP-49 ligand, and other potential ligands) in driving predator-mediated behavioral changes. The underlying neural circuits will be mapped. In Aim 2, the mechanism by which CRF signaling in neurons is relayed to other tissues, resulting in the induction of mitochondrial stress, will be determined. In Aim 3, a focused genetic screen will be performed to identify additional components of the CRF signaling pathway that are responsible for stress-induced behavioral and physiological changes. These studies will reveal how neural circuits and the CRF signaling pathway process information about environmental threats to generate adaptive stress responses.